1. Technical Field
The present invention is related to a clutch disconnection/connection device, and more particularly to a clutch disconnection/connection device that is capable of automatic clutch operation of a vehicle.
2. Background Art
Recently, the demand for automatic transmission is increasing not only in small vehicles but also in large vehicles such as buses and trucks. Since a fluid-type torque converter adopted as a clutch type in smaller cars is disadvantageous when it is used for large vehicles of heavy weight and large load because it increases their fuel consumption, in case of large vehicles, automatic transmission is normally achieved by disconnecting/connecting friction clutches through automatic operation and then transferring the output to a transmission that is automatically operated. A clutch disconnecting/connecting device of this kind carrying out automatic clutch operation is generally provided with a clutch booster that disconnects/connects the friction clutches by supplying/discharging air pressure.
However, as a quite delicate operation is required at starting the vehicle or some other occasions, if the clutch disconnection/connection device tries to maintain its automatic clutch control even in such tricky situations, it inevitably has to have a complicated and thus costly design. Thus, another type of clutching disconnection/connection device has been proposed that is basically automatic but allows manual operation by a clutch pedal only when a delicate clutch operation is required (what is called a semi-automatic clutch system). The semi-automatic clutch system has an advantage that its construction is simpler and less costly. In the semiautomatic clutch system, a hydraulic pressure is supplied from/discharged to a master cylinder by operating the clutch pedal, and air pressure is supplied to/discharged from the clutch booster by this supplying/discharging of hydraulic pressure.
By the way, air pressure is supplied to/discharged from the clutch booster during automatic transmission(except for starting the vehicle) regardless of operation of the clutch pedal. In addition, the clutch booster is constructed as to operatively disconnect/connect the clutch by pushing an internal power piston when the air pressure is supplied.
In the conventional device mentioned above, a passage for distributing the hydraulic pressure from the master cylinder communicates with an hydraulic cylinder(of the booster) that changes its volume in accordance with the movement of the power piston. Therefore, during the automatic disconnection/connection control of the clutch, that is, when the disconnection/connection control of the clutch is performed by the power piston without operating the clutch pedal, movement of the hydraulic piston due to the pushing of the power piston could cause a negative pressure inside the hydraulic pressure distribution passage and introduction of air bubbles into the passage, making an accurate operation of the clutch impossible.
In order to prevent the occurrence of such negative pressure as above, Japanese Utility Model No. 4-8023 and others suggest providing a cancellation mechanism of manual operation and automatic operation in an hydraulic output part of the clutch booster, such that change in volume of the hydraulic pressure distribution passage can be prevented during the automatic operation. However, this kind of structural modification of the clutch booster ends up with complicated arrangements(necessary for reliable sealing) in a relatively small space, resulting in not only higher cost but also less reliability and perseverance of the device as a whole.
Thus, for solving the problems as above, the applicant of the present invention has previously proposed a clutch disconnection/connection device in which the structure of the clutch booster is not modified but the master cylinder can be operated by another drive means(air pressure or hydraulic pressure), as well as by the clutch pedal.
One example of such a new type device is described in Japanese Patent Application 7-205859. That device supplies an air pressure to the master cylinder in an automatic disconnection operation of the clutch and pushes a piston with the air pressure in accordance with the clutch-disconnection operation such that an hydraulic pressure is generated from the master cylinder. Due to this, the occurrence of negative pressure inside the hydraulic pressure distribution passage can reliably prevented.
In such a conventional device as above, the manual operation is preferentially performed to the automatic operation. That is, even when the clutch is automatically operated, if the driver steps the clutch pedal, the automatic operation is discontinued and switched to the manual operation.
However, as this transition(shift) is carried out after a clutch pedal switch has detected the driver's stepping the clutch pedal and a relatively slight stroke(stepping) of the clutch pedal is sensitively detected by the clutch pedal switch, if the clutch pedal is stroked(stepped) when the clutch is completely disconnected, the clutch may rapidly move to a position corresponding to the state of the stroked clutch pedal, causing a relatively big clutch-connection shock, especially at a low-speed gear operation.
Some modifications, such as detecting the stroke of the clutch pedal at a larger stroke(a more disconnecting stroke) or slowing the speed at which the clutch returns when the automatic operation is discontinued has been proposed. But none of them provides the driver an ideal clutch operation as he/she desires.